Temporal aspects of the abrasion of microphytic crusts under grain impact

Wind-tunnel simulations of the response of two moss crusts to grain impact indicate that, given sufficient time, these surfaces will deteriorate under very low wind velocities only slightly above u(st) for the loose, saltating grains. In parallel with these experiments, the frequency distributions of ultimate strength and penetration energy were determined for each of the two crust types via penetrometry. Pohlia was found to be stronger than Tortula; but, even so, both of these crusts had ultimate strengths 20-350 times higher than the force delivered by a single grain impacting each surface at a velocity of 1 ms(-1). In comparison, the modulus of deformation and penetration energy data were very similar for the two surface types, especially for the weakest areas of crust development. This observation is in accord with the wind-tunnel simulations that also found no consistent difference in the response of these two crust types to impact. In comparison with crusts formed by clay and salt, fibrous microphytic crusts are morphologically complex and typically weak. The notable elasticity of these surfaces does reduce the force of grain impact, and thereby provides some protection against rupture. One of the central conclusions of this study suggests that not only is the particle kinetic energy at impact important in crust breakdown, but also tiny fractures at points of localized stress concentration contribute to a progressive reduction in the integrity of the filament net. In some of the experiments conducted as part of this study, up to 50 or more minutes of constant bombardment was required to produce small abrasion marks on selected areas of the microphytic crust. This study prepares a foundation for future experiments needed to examine the breakdown of complex crusts formed in nature. Copyright (C) 2002 John Wiley Sons, Ltd.